Is the current rise in the atmospheric concentration of carbon dioxide causing global warming? Michael Crichton, in the novel “State of Fear” says no. The vast majority of scientists disagree, but science is not democratic. (Scientists do not conduct referenda to resolve their frequent disputes.) Al Gore, in “An Inconvenient Truth” advances the debate by shifting attention from developments over the past 100 years, to the recurrent Ice Ages of the last 400,000 years. Unfortunately the role of atmospheric carbon dioxide variations in the Ice Age cycle is unclear. Fortunately, much longer records, of temperature changes since the demise of the dinosaurs some 65 million years ago provide a valuable perspective on our current activities: the present is a precarious moment in the history of our planet, one that calls for circumspection on the part of those who call themselves Homo sapiens.